The present invention relates generally to the field of media labeling. In particular, the invention relates to methods and systems for correcting for the change in optical characteristics which occur when a disc is subject to label writing as different from data writing/reading.
Digital media, such as compact discs (CDs) or digital video discs (DVDs), are a popular form of storage media. Recently, writable digital media have become increasingly popular among users for storing personalized data, including creating their own set of musical compilations, pictures, etc. Once the user has stored or written digital data onto the medium, the user may label the medium by either writing on the medium by hand or affixing a printed label onto the medium using an adhesive.
More recently, systems have been developed for imprinting a labeling layer on a digital medium using a read/write laser of the disc drive. In such systems, laser energy is applied to change the color or darkness of desired locations on the labeling layer so as to produce an image.
In such systems, the same optical pickup unit (OPU) may be used both for writing data to a data layer of the digital medium, and for writing a label to the labeling layer. Writing to the data layer requires passing the laser energy from the OPU through a substrate layer such as transparent polycarbonate, acrylate or glass. The OPU is typically designed for writing to the data layer, and thus the optics used in writing data are pre-corrected for the spherical aberration introduced by the transparent substrate of a typical thickness. However, in writing to the labeling layer the laser energy passes either primarily through air, or through a smaller amount of transparent material, rather than through the same thickness of the substrate as when writing to the data layer. As a result, when the optical path designed for writing to the data layer is also used to write a label, a strong spherical aberration of the opposite sign is introduced as the optics are focused through the air or the smaller thickness of transparent material This aberration causes some of the optical sensor functions in the drive to operate improperly. This results in difficulty in using the expected and usual methods for servo feedback for positioning the objective lens for focus.